The invention relates to a process for the production of substantially pure methallylbenzene, isobutenylbenzene, and their p-substituted alkyl derivatives of Formula 1 and 2, respectively, by a process for the thermal cracking of neophyl chloride and the corresponding p-substituted neophyl chlorides of Formula 3, and if desired a subsequent isomerization of the cracked .beta.-olefins of Formula 2 to the corresponding .alpha.-olefin of Formula 2, while undergoing fractional distillation. ##STR2##